The present invention relates to an apparatus and method for planning and guiding insertion of an object along a linear trajectory into a body. More particularly, the present invention relates to an apparatus and method for coordinating two captured fluoroscope images to permit effective three-dimensional planning of the trajectory using only two-dimensional images.
Numerous medical interventions involve placing a needle, drill, screw, nail, wire or other device in the body. In some cases the angle and position of the device are both of critical importance, for example in the drilling of a hole for a screw along the axis of a spiral pedicle. In other cases, it is primarily the positioning of the end-point of the device which is important, for example in placing a biopsy needle into a suspected tumor. In still other cases, the objective is only to define a point rather than a line, for example in targeting a tumor for radiation therapy. Many other examples exist, especially in the field of orthopaedics.
The present invention is also relevant to the development of percutaneous technique. Executing a linear trajectory for the insertion of instrumentation into the body through the skin is more difficult than open surgical technique, but the reduced invasiveness and trauma of percutaneous placement makes it desirable.
Fluoroscopy is frequently used by surgeons to assist medical procedures. Continuous fluoroscopy during a surgical procedure is undesirable because it exposes the surgeon's hands to radiation. Furthermore, regardless of whether intermittent or continuous fluoroscopy is used, the resulting images are two-dimensional while insertion of the surgical instrument requires three-dimensional awareness by the surgeon.
The apparatus and method of the present invention involve acquisition and storage of two separate fluoroscopic images of the body, taken from two different angles. Typically, although not necessarily, these would be an anterior/posterior (A/P) image taken front-to-back of the patient, and a sagittal image taken side-to-side. These two fluoroscopic images are displayed on two adjacent computer monitors. The surgeon uses a trackball or other computer input device to specify on the monitors an insertion point and an insertion trajectory.
A mechanical positioning device is then used to position a guide through which the surgeon performs the insertion of the surgical instrument. The positioning device may either be an active computer controlled manipulator such as a robot, or it may be a manually adjusted mechanical device which is set numerically in accordance with an output from the computer.
The apparatus and method of the present invention establish the projective geometric relationship relating each of two acquired fluoroscopic images to the three-dimensional workspace around and within the patient's body, despite essentially arbitrary positioning of the fluoroscope. The two images then become a coordinated pair, which permits three-dimensional planning that might otherwise be expected to require a computed tomography (CT) scan.
While the acquisition and display of two approximately orthogonal images may be expected to present the surgeon with the greatest ability to plan in three dimensions, two images are not strictly necessary. It is possible to use a single captured image for some procedures, particularly if the surgeon has adjusted the beam axis of the fluoroscope into alignment with the intended trajectory. Furthermore, more than two images could also be acquired and coordinated, should that be advantageous.
Several other approaches to stereotactic or robotic surgery, planned on a computer screen displaying medical images, have been described by other workers, and will be listed below. Some background is given here before discussing prior art. The method and apparatus of the present invention constitute a technique we call coordinated fluoroscopy. Coordinate fluoroscopy is a technique for REGISTRATION and for SURGICAL PLANNING. It allows registration based on the acquired fluoroscopic images themselves, without requiring any additional measuring devices. It allows three-dimensional surgical planning based on fluoroscopic views from two angles, without requiring three-dimensional imaging such as computed tomography (CT), and without requiring that the two fluoroscopic images be acquired from orthogonal fluoroscope poses.
Registration
Registration is a key step in any image-guided surgical system. Registration is the determination of the correspondence between points of the image upon which a surgical plan is prepared, and points of the workspace in the vicinity of (and within) the patient. If a numerically controlled tool (whether robotic or manual) is to be used, the coordinate system of that device must also be brought into registry with the image.
It is common to accomplish registration with the help of a global positioning device, usually optical, which can measure the three-dimensional coordinates of markers placed anywhere over a large volume of space. Coordinated fluoroscopy avoids the necessity for this expensive and inconvenient device, instead deriving registration directly from the acquired fluoroscopic images themselves. Coordinated fluoroscopy uses a “registration artifact” which is held in a fixed position relative to the patient while one or more fluoroscopic images are acquired from different angles (poses). There is no need to constrain the fluoroscope poses at which these various images are acquired, for instance to require that they be orthogonal, nor is there a need to instrument the fluoroscope so that the pose angles can be measures. Instead, pose information is extracted after-the-fact from the images. It is a substantial benefit of the present invention that surgeons can acquire fluoroscopic images using fluoroscope poses of their own choosing, as they are accustomed.
The registration artifact contains a plurality of features (fiducials) which are designed to be easily identifiable on a fluoroscopic image. The embodiment described here uses eight small steel spheres embedded in a radiolucent matrix. The positions of these fiducials are known relative to a coordinate system fixed in the artifact, either by design or by measurement.
From the two-dimensional locations of the projections of these fiducials in a fluoroscopic image, we can determine the geometric projections that carry a general three dimensional point anywhere in the vicinity of the artifact into a projected point on the image. This establishes registration between image and workspace. Several images can each be registered relative to the same registration artifact, thus also bringing all the images into registry with one another.
Identification of the geometric projections, as discussed above, would not be possible with raw fluoroscope images, which are highly nonlinear and distorted. It is necessary first to map and compensate for these distortions. It is useful to be aware of the necessity of distortion compensation when comparing the present invention to prior art.
Surgical Planning
Surgical planning is also a key step in image-guided surgery. Planning of three-dimensional surgical procedures might be expected to be done on a three-dimensional dataset, such as can be reconstructed from computed tomography (CT) data. However, surgeons are accustomed to planning on two-dimensional images: radiographs or fluoroscopic images. Indeed even when CT data is available, planning is usually done on individual two-dimensional CT “slices” rather than on a three-dimensional reconstruction.
The coordinates of the endpoints of a line segment representing an intended screw, biopsy needle, or drilled hole are of course three-dimensional, as are the coordinates of a single point within the body marking the present location of a tumor or a fragment of shrapnel. In surgical planning such points can be specified on a two-dimensional image, or on each of several two-dimensional images. Each such two-dimensional image is a projection of the same three-dimensional space.
It is necessary to convert the two-dimensional coordinates of specified points on each of several images into a three-dimensional coordinate which can be used to guide a tool along a desired trajectory or to a desired point within the body. To do so one must have knowledge of the geometric relationship of the projections that created the images.
In the absence of such geometric knowledge a point specified on one image and a point independently specified on another image may in fact not correspond to any single point within the body. This is so because a point specified on a two-dimensional image is the projection of a LINE in space. The implied point in three-dimensions is the intersection of two such lines, one implied by the point specified on each image. Two such lines created independently may be skew, intersecting nowhere. Similarly, line segments for an intended procedure can not be chosen independently on two images, otherwise they will in general not correspond to a well-defined three-dimensional line segment.
In coordinated fluoroscopy, the geometric projections that relate the two images to a single three-dimensional coordinate system are established before planning commences. The points chosen by the surgeon on two (or more) images can therefore be constrained by the software such that they DO correspond to a well-defined point in three-dimensions. In practice, as a surgeon adjusts an intended point or line segment on one image, the point or line segment displayed on the other image(s) continuously updates and adjusts as well. One cannot draw “arbitrary” points or line segments independently on the images; the software only allows one to draw points or line segments that correspond to a well-defined point or line segment in three-dimensions.
The benefits of planning on geometrically coordinated images as described above are threefold:                1) Once the surgeon has selected a point or a line segment on two images, the three-dimensional point or line segment to which the selections correspond is fully defined and ready to be executed.        2) An axial view such as could be attained from a CT slice is generally unattainable fluoroscpically. The angle that is most easily visualized in axial view, known as the transverse angle, is therefore difficult to select or execute under fluoroscopy. In coordinated fluoroscopy the transverse angle is implicitly specified by the surgeon by selecting line segments on two images. This may assist the surgeon in visualizing and planning the transverse angle for a procedure.        3) In conventional fluoroscopy, image dilation due to beam divergence is of unknown extent, making accurate measurement of anatomic distances difficult. In coordinated fluoroscopy the actual in-situ length of an intended line segment can be determined by the software. This is useful for selecting appropriate screw length, as well as for other purposes.        